Cables are omnipresent in all electrical systems, for supplying or transmitting information. These cables are subject to the same stresses as the systems that they link and can be subject to failures. It is therefore necessary to be able to analyze their condition and provide information on the detection of faults, but also the location and type thereof, in order to assist in maintenance. The standard reflectometry methods allow for this type of testing.
The reflectometry methods use a principle similar to that of radar: an electrical signal, the probe signal or reference signal, which is more often than not high-frequency or wideband, is injected at one or more points of the cable to be tested. The signal is propagated in the cable or the network and returns a portion of its energy when it encounters an electrical discontinuity. An electrical discontinuity can result, for example, from a connection, from the end of the cable or from a fault or, more generally, from a break in the conditions of propagation of the signal in the cable. It results more often than not from a fault which locally alters the characteristic impedance of the cable by provoking a discontinuity in its line parameters.
The analysis of the signals returned to the injection point makes it possible to deduce therefrom information on the presence and on the location of these discontinuities, therefore of any faults. An analysis in the time or frequency domain is usually performed. These methods are referred to by the acronyms TDR, from the expression “time domain reflectometry”, and FDR, from the expression “frequency domain reflectometry”.
The invention falls within the scope of the reflectometry methods for wired diagnostics and applies to any type of electrical cable, in particular power transmission cables or communication cables, in fixed or mobile installations. The cables concerned can be coaxial, two-wired, in parallel lines, in twisted pairs or the like, provided that it is possible to inject therein a reflectometry signal at a point of the cable and measure its reflection at the same point or at another point.
The known time-domain reflectometry methods are particularly suited to the detection of hard faults in a cable, such as a short-circuit or an open-circuit or, more generally, a significant local alteration of the impedance of the cable. The detection of the fault is done by measuring the amplitude of the signal reflected on this fault which is all the greater and therefore detectable when the fault is significant.
Conversely, a soft fault, for example resulting from a superficial degradation of the cladding of the cable, of the insulation or of the conductor, generates a peak of low amplitude on the reflected reflectometry signal and is consequently more difficult to detect by conventional time-domain methods. More generally, a soft fault can be provoked by a friction, a pinching or even a corrosion phenomenon affecting the cladding of the cable, the insulation or the conductor.
The detection and the location of a soft fault on a cable is a major problem for the industrial world because a fault generally appears first as a superficial fault but can, over time, evolve to a fault with greater impact. For this reason in particular, it is useful to be able to detect the appearance of a fault as soon as it appears and at a stage where its impact is superficial in order to anticipate its evolution to a greater fault.
The low amplitude of the reflections associated with the passage of the signal through a soft fault also creates a potential false detection problem. Indeed, it can be difficult to discriminate a peak of low amplitude in a reflectogram which may result either from a fault on the cable, or from a measurement noise. Thus, false positives can appear which do not correspond to faults but which result from the measurement noise or from non-uniformities of the cable.
The prior art includes several methods for detecting soft faults based on the principle of reflectometry.
The French patent application from the Applicant published under the number FR2981752 proposes a method for post-processing of a reflectogram based on the mathematical Wigner-Ville transform. The method consists of a time-frequency analysis of the reflectogram in order to emphasize the components corresponding to the faults sought. One drawback with this method is its high complexity for embedded equipment.
The French patent application from the Applicant published under the number FR3006769 proposes a different approach based on a processing of the signal measured in the frequency domain in order to estimate certain characteristic parameters of the propagation of the signal in the cable. The detection of soft faults is done by analyzing the parameters estimated.
Also known are the French patent applications from the Applicant filed respectively under the numbers FR1459402 and FR1554632. The first proposes a method for auto-adaptive correlation between the reflected signal and the injected signal while the second focuses on a search for zero-crossings of the reflectogram and a comparison of the power of the signal calculated on either side of the zero-crossing. The method called subtractive correlation described in the document “Méthodes d′amélioration pour le diagnostic de câble par réflectométrie” [Enhancement methods for cable diagnostics by reflectometry], Lola El Sahmarany, Université Blaise Pascal—Clermont-Ferrand II, 2013, can also be cited.
These latter methods offer better results for the detection of soft faults but can however generate a not-inconsiderable number of false detections or false positives because of the difficulty in discriminating the peaks of low amplitude associated with the faults and those associated with measurement noise or with inherent non-uniformities of the cable.
The invention proposes a novel method for detecting soft faults which is based on a post-processing applied to the integral of the measured reflectogram.